1. Technical Field
The inventive arrangements relate generally to methods and apparatus providing advanced operating features for programs recorded on disc media, for example recordable digital video discs, hard disk drives and optical discs.
2. Description of the Related Art
While conventional trick modes enable a user to view recorded video at different speeds, the video is not permanently altered by the process. If the video is displayed at a later time, then the user must initiate another trick mode to view the video at a different speed. Significantly, however, many users may wish to permanently change the playback speed of a particular portion of video to achieve special visual effects by modifying the video once the video has been recorded onto the media. Such a process can permit speed variations during playback. Unfortunately, several significant obstacles exist that make editing recorded video in such a fashion impracticable.
For example in the case of media such as magnetic tape and optical disc media such as DVD, it is desirable to record program data in a contiguous manner on the medium so as to avoid interruptions in program presentation. Typically, however, the space occupied by the original recorded video is not large enough to store the pictures that must be repeated in order to produce certain trick effects such as slow motion playback. This makes it difficult to store a modified video segment in the same physical location as the original data. In addition to spatial limitations, repeating pictures on the disc or other media can interfere with the conventional picture structure of the video, which can degrade the display quality of the video during playback.
Performing trick modes on video recorded under a film structure format, particularly when a moving object is recorded, can present another problem as well. Video recorded under a film structure format is recorded at twenty-four (24) frames per second. However, video displayed on televisions is typically displayed at a rate of about thirty (30) frames per second or, equivalently, at an NTSC rate of sixty (60) fields per second when interlaced scanning is used. Based on the incompatibility of the display rates between film and television, DVD players or recorders can convert video recorded under a film structure to an NTSC format using a process known as 3:2 pulldown.
Using 3:2 pulldown, a DVD player or recorder will rearrange each frame into a top field and a bottom field. For example, an initial frame from a group of four (4) is shown as three (3) fields, where the top field is shown first, then the bottom field is shown second, then the top field is shown again. The next film frame is shown as two (2) fields, with the bottom field being shown first and the top field being shown second. The third film frame of the group is again shown as three (3) fields, but in this frame the bottom field is shown first, followed by the top field, and the bottom field is shown again. Lastly, the fourth frame is shown as two fields, with the top field shown first and the bottom field shown last. Since 3:2 pulldown adds repeat fields to a video during playback and modifying recorded video to produce slow motion playback adds additional repeat pictures, inconsistent object movement is likely to result if the pictures repeated for slow motion playback contain repeat fields. Thus, a need exists for a method of changing a playback speed for video recorded in a modified film format having repeat fields and further overcoming the detriments described above.
In a first aspect of the invention, a method for changing the playback speed of a selected video segment having repeat fields includes the step of modifying the selected video segment by removing repeat fields to achieve fast motion play. This can be accomplished by setting a repeat first field flag located in a picture header to zero to prevent 3:2 pulldown from being performed on a video segment. Pictures from the selected video segment can also be removed to achieve a desired playback speed.
In a second aspect of the invention, the method can also include the step of encoding the repeat fields into conventional field pictures. An associated repeat first field flag located in a picture header can then be set to zero. Dummy pictures can be added to the selected video segment to achieve a desired playback speed.
Yet another aspect of the present invention for modifying a selected video segment having repeat fields includes the steps of replacing the repeat fields with dummy pictures and setting a repeat first field flag located in a picture header to zero. Further dummy pictures can be added to the selected video segment to achieve a desired playback speed. In the above discussed methods, the selected video segment can be reencoded into a conventional or other desired GOP structure after modifying the selected video segment.
Another aspect of the present invention comprises a system for changing the playback speed of a selected video segment having repeat fields. The system can include storage medium reading circuitry for selectively reading a video segment which has been recorded on a portion of a rewritable storage medium, a video processor for modifying the selected video segment for a changed playback speed, and video recorder circuitry for recording the modified video segment exclusively on a portion of the storage medium. The video processor can modify the selected video segment by removing the repeat fields to achieve fast motion play. The video processor can also modify the selected video segment by encoding the repeat fields into conventional field pictures and/or setting a repeat first field flag located in a video header to zero after modifying a selected video segment.
The video processor may also delete a plurality of non-video packs in the selected video segment to reduce an amount of data contained in the modified video segment. Furthermore, the video processor can reduce the resolution of at least one frame contained in the modified video segment and/or lower the bit rate of the modified video segment during a recording step. In a video segment, the video processor can add or delete at least one dummy picture to change the playback speed.
In the above system, a selected video segment can be reencoded into a desired GOP structure after being modified and the modified segment can be recorded exclusively on a desired portion of a storage medium.